Aromatic polyimides obtained by polymerization of aromatic tetracarboxylic acid compounds and aromatic diamine compounds are excellent in mechanical strength, heat resistance, electrical insulating properties, chemical resistance, dimensional stability, etc. and are widely used particularly for electronic device materials.
In recent years, applications of aromatic polyimides to semiconductor materials and thin-film solar cell materials are growing. Substrate materials such as silicon used for these materials have low thermal expansivity, and high-temperature treatment such as sputtering is required in the production processes of these materials.
Therefore, polyidmides used for the above purposes are required to have higher dimensional stability and heat resistance than conventional polyimides. Specifically, the thermal expansion coefficient, which is a measure of dimensional stability, of these polyimides must be 10 ppm/° C. or lower, and their thermal decomposition temperature, which is a measure of heat resistance, must be 600° C. or higher.
However, aromatic polyimides have poor solubility in solvent because of their rigid molecular structure and strong interaction of imide bonds providing linkages in the molecular structure. To form an aromatic polyimide into a shape, a polyamide acid varnish that is a precursor of the polyimide must be used. Hence good solubility of polyamide acid or polyamide acid composition in solvent is required about the polyamide acid varnish (hereinafter solubility of polyamide acid or polyamide acid composition in solvent is referred to as varnish solubility).
Examples of the conventional polyimides used widely include: two-component polyimides such as pyromellitic acid dianhydride (PMDA)-4,4′-diaminodiphenyl ether (ODA)-based polyimides shown in Patent Literature 1; and 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (sBPDA)-p-phenylenediamine (PDA)-based polyimides shown in Patent Literature 2. However, the thermal decomposition temperature of the PMDA-ODA-based polyimides is lower than 600° C., and their thermal expansion coefficient is high, about 40 ppm/° C. The heat resistance of the eBPDA-PDA-based polyimides is higher than that of the PMDA-ODA-based polyimides, and the thermal expansion coefficient of the sBPDA-PDA-based polyimides is lower than that of the PMDA-ODA-based polyimides. However, these do not meet the characteristics required for the above-described purposes.
Many three- and four-component polyimides prepared by copolymerization of the above-described monomers at given ratios have been proposed in, for example, Patent Literature 3. However, these polyimides tend to have lower heat resistance and a higher thermal expansion coefficient than the sBPDA-PDA-based polyimides.
Attempts to obtain polyimides having a low thermal expansion coefficient and high heat resistance are also shown in Patent Literature 4 to Patent Literature 10.
The polyimide described in Patent Literature 4 is obtained by using 2,3,6,7-naphthalenetetracarboxylic acid dianhydride (NTCDA) and an aromatic diamine component having a specific structure.
Patent Literature 5 uses, as a polyimide precursor that forms a polyimide having a 5% thermal weight loss temperature of 500° C. or higher, a compound obtained by reacting specified total diamines and specified total tetracarboxylic acid dianhydrides including NTCDA.
The polyimide described in Patent Literature 6 uses a specific aromatic diamine as a diamine component.
The polyimide described and used in Patent Literature 7 is obtained by polymerization of a specific acid dianhydride component including NTCDA and specific diamine components including p-phenylenediamine (PDA).
The polyimide described and used in Patent Literature 8 is obtained by causing a reaction between a specific aromatic diamine including PDA and a specific aromatic tetracarboxylic acid dianhydride including NTCDA.
The polyimide described and used in Patent Literature 9 is derived from a specific aromatic dianhydride including 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (sBPDA) and a specific aromatic diamine including 2,2′-bis(trifluromethyl)benzidine (TFMB).
The polyimide described and used in Patent Literature 10 is obtained by polymerization of diamine components including PDA and TFMB and an acid component including sBPDA.
However, the polyimides described in Patent Literature 4 to Patent Literature 10 cannot meet the above-described requirement on the dimensional stability (thermal expansion coefficient: 10 ppm/° C. or lower), the requirement on the heat resistance (thermal decomposition temperature: 600° C. or higher), and the requirement on the varnish solubility.